The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. xc2xa7202) in which the Contractor has elected to retain title.
1. Technical Field
This invention relates to imaging systems, and more particularly to Quantum Well Infrared Photodetectors.
2. Background
Infrared imaging is widely used in a variety of applications including night vision, surveillance, search and rescue, remote sensing, and preventive maintenance, to name a few. Imaging devices to provide these applications are typically constructed of HgCdTe or InSb focal plane arrays. These focal plane arrays are known to be pixel mapped devices, where an array element is generally mapped to one or more circuit elements. However, such focal plane arrays are difficult to manufacture and expensive. Quantum Well Infrared Photodetectors (QWIPs) can detect mid and far infrared light, providing an output current as a result.
GaAs based Quantum Well Infrared Photodetectors are useful for several applications such as target recognition and discrimination which require mid-wavelength:long-wavelength, long-wavelength:long-wavelength, and long-wavelength:very-long wavelength large area, uniform, reproducible, low cost and stable multi-color infrared focal plane arrays (FPAs). For example, a two-color FPA camera would provide the absolute temperature of a target which is extremely important to the process of identifying temperature difference between targets, war heads and decoys. On the other-hand, two-color is not sufficient in identifying the absolute temperature of objects in the presence of a third variable such as the Earth""s reflection in exo-atmospheric applications. Thus, three-color FPAs are more suitable for exo-atmospheric target recognition applications.
Random reflectors are potentially broadband optical coupling structures for QWIPs. When combined with a thinned QWIP substrate, the random surface can trap the optical field by reflecting the waves at different angles on each bounce. For efficient coupling to the absorbing QW layer, the surface should (1) diffract efficiently into high angles, and (2) have near-zero diffraction efficiency at low angles. Condition 1 can be produced by a surface that has significant depth variation on the scale of one wavelength. Condition 2 can be produced by a surface that produces destructive interference of all reflected waves in the direction normal to the surface. To produce a broadband optical coupling structure, both conditions need to be satisfied over a significant wavelength range. Zeroing the normal reflection, however, is the more important condition since any normally reflected light is lost without any absorption. The structures described here can produce a minimal normal reflection over a significant wavelength range. In all the simulations, scalar electromagnetic theory is used, and this is only an approximation when the wavelength is on the order of the feature size.
Imaging systems that operate in the very long wavelength infrared (VLWIR) region are required in a variety of NASA""s earth science applications, such as geological and volconological studies, monitoring global atmospheric temperature profiles, cloud characteristics, and relative humidity profiles. This is mainly due to the fact that most absorption lines of atmospheric gas molecules such as ozone, water, carbon dioxide, carbon monoxide, sulfur dioxide, and nitrous oxide occur in the LWIR spectral range. In addition, 12-18 xcexcm focal plane arrays (FPAs) would be very useful in detecting cold objects such as ballistic missiles in midcourse (when hot rocket engine is not burning most of the emission peaks are in the 8-15 xcexcm IR region). Thus, it is desirable to develop highly sensitive, low power dissipation, large, VLWIR FPAs which can simplify the design and construction of infrared imaging systems.
Spectral response of conventional interband infrared (IR) detectors are completely determined by the bandgap because photoexcitation occurs across the band gap (Eg) from the valence to conduction band. Therefore, detection of very is long wavelength IR radiation requires small bandgap materials such as Hglxe2x88x92xCdxTe and Pblxe2x88x92xSnxTe, in which the energy gap can be controlled by varying x. It is well known that these low band gap materials are more difficult to grow and process than large band gap semiconductors such as GaAs. Although, these detectors in single element format show high performances at higher operating temperatures, it is extremely difficult to produce them in large format uniform arrays.
Quantum Well Infrared Photodetectors avoid such difficulties because they are fabricated using high bandgap materials systems such as GaAs/AlxGa1xe2x88x92xAs. The detection mechanism of QWIP involves photoexcitation of electrons between ground and first excited states (subbands) of the quantum well which is created in the conduction band due to bandgap difference in the material system. Quantum well parameters for GaAs/AlxGa1xe2x88x92xAs material systems can be designed to detect light at any wavelength from 6 to 25 xcexcm range. The advantages of QWIPs compared with HgCdTe detectors include high uniformity, excellent reproducibility, low 1/f noise and low-cost large-area staring arrays. However, it is difficult to obtain this signal to noise ratio for VLWIR QWIPs at high operating temperatures. This is due to high dark current which is dominated by classical thermionic emission at such operating temperatures.
The present invention includes a three-color QWIP focal plane array. The three-color QWIP focal plane array is based on a GaAs/AlGaAs material system. Three-color QWIPs enable target recognition and discriminating systems to precisely obtain the temperature of two objects in the presence of a third unknown parameter. The QWIPs are designed to reduce the normal reflection over a significant wavelength range.
One aspect of the present invention involves two photon absorptions per transition in a double quantum well structure which is different from typical QWIP structures. This design is expected to significantly reduce the dark current as a result of higher thermionic barriers and therefore allow the devices to operate at elevated temperatures. The device is expected to be fabricated using a GaAs/AlxGa1xe2x88x92xAs material system on a semi-insulating GaAs substrate by Molecular Beam Epitacy (MBE).